1. Adjust oven rack to middle position; heat oven to 375°F. Melt 4 tablespoons butter in medium saucepan over medium heat. Add cocoa powder and espresso powder; stir until mixture forms smooth paste. Set aside to cool, 15 to 20 minutes.

2. In standing mixer fitted with paddle attachment, mix remaining 16 tablespoons butter, sugar, salt, and cooled cocoa mixture on high speed until well combined and fluffy, about 1 minute, scraping sides of mixing bowl once or twice with rubber spatula. Add yolks and vanilla and mix on medium speed until thoroughly combined, about 30 seconds. Scrape sides of bowl. With mixer running on low, add flour in three additions, waiting until each addition is incorporated before adding next and scraping bowl after each addition. Continue to mix until dough forms cohesive ball, about 5 seconds. Turn dough onto counter; divide into three 4-inch disks. Wrap each disk in plastic wrap and refrigerate until dough is firm yet malleable, 45 to 60 minutes. (Alternatively, shape dough log, 2 inches in diameter and about 12 inches long; use parchment paper or plastic wrap to roll into neat cylinder. Chill until very firm and cold, at least 1 hour.)

3. Roll out 1 dough disk between 2 large sheets parchment paper to even thickness of 3/16 inches. (If dough becomes soft and sticky, slide rolled dough on parchment onto baking sheet and rechill until firm, about 10 minutes.) Peel parchment from one side of dough and cut into desired shapes using cookie cutters; using thin metal spatula, place shapes on parchment-lined baking sheet, spacing them about 1 inch apart. Gather dough scraps and chill. (For cylinder-shaped dough, simply slice cookies 1/4-inch-thick and place on parchment-lined baking sheets.)

4. Bake until cookies show slight resistance to touch (see photo below), 10 to 12 minutes, rotating baking sheet halfway through baking time; if cookies begin to darken on edges, they have overbaked. Cool for 5 minutes, then, using spatula, transfer cookies to wire rack; cool completely. Repeat steps 3 and 4 with remaining dough disks and scraps, rerolling scraps just once. Decorate as desired.

5. For Glaze: Melt bittersweet chocolate with butter and whisk until smooth. Add corn syrup and vanilla extract and mix until smooth and shiny. Use back of spoon to spread scant 1 teaspoon glaze almost to edge of each cookie. (If necessary, reheat to prolong fluidity of glaze.) Allow glazed cookies to dry at least 20 minutes.

Makes about 4 dozen 2 1/2-inch cookies